Distributed Base Station with Frequency Domain Interface on Which Signal Subspace Varies According to Frequency Bin

ABSTRACT

A method and apparatus is disclosed for determining a signal subspace in a communications system. A remote apparatus obtains signal streams from antenna elements or signal streams from antenna beams. Based on the obtained signal streams, the apparatus selects a signal subspace for a user, the signal sub-space having a dimension M. Based on the selected signal subspace, the apparatus transmits, via an interface to a central apparatus, M streams of post-fast-Fourier-transform data, the interface being capable of transmitting a different sub-space for different frequency bins.

FIELD OF THE INVENTION

The exemplary and non-limiting embodiments of this invention relategenerally to wireless communications networks, and more particularly todetermining a signal subspace.

BACKGROUND ART

The following description of background art may include insights,discoveries, understandings or disclosures, or associations togetherwith dis-closures not known to the relevant art prior to the presentinvention but provided by the invention. Some such contributions of theinvention may be specifically pointed out below, whereas other suchcontributions of the invention will be apparent from their context.

In antenna array architectures for an OFDM (orthogonal frequencydivision multiplexing) system, it is possible for post-FFT (fast Fouriertransform) array processing to reduce the complexity of the antennasystem by decreasing the number of antennas and the correspondingcomponents, because its performance depends on the number of signalsources. The post-FFT array processing is able to achieve highperformance with lower complexity by using subcarrier clustering.

SUMMARY

The following presents a simplified summary of the invention in order toprovide a basic understanding of some aspects of the invention. Thissummary is not an extensive overview of the invention. It is notintended to identify key/critical elements of the invention or todelineate the scope of the invention. Its sole purpose is to presentsome concepts of the invention in a simplified form as a prelude to themore detailed description that is presented later.

Various aspects of the invention comprise a method, apparatus, basestation, and a computer program product as defined in the independentclaims. Further embodiments of the invention are disclosed in thedependent claims.

An aspect of the invention relates to a method for determining a signalsubspace in a communications system, the method comprising obtaining, ina remote apparatus, signal streams from antenna elements or signalstreams from antenna beams; based on the obtained signal streams,selecting a signal subspace for a user, the signal subspace having adimension M; and based on the selected signal subspace, transmitting,via an interface to a central apparatus, M streams ofpost-fast-Fourier-transform data, the interface being capable oftransmitting a different subspace for different frequency bins.

A further aspect of the invention relates to an apparatus comprising atleast one processor; and at least one memory including a computerprogram code, wherein the at least one memory and the computer programcode are configured to, with the at least one processor, cause theapparatus to obtain signal streams from antenna elements or signalstreams from antenna beams; based on the obtained signal streams, selecta signal subspace for a user, the signal subspace having a dimension M;and based on the selected signal subspace, transmit, via an interface toa central apparatus, M streams of post-fast-Fourier-transform data, theinterface being capable of transmitting a different subspace fordifferent frequency bins.

A still further aspect of the invention relates to a base stationcomprising the apparatus.

A still further aspect of the invention relates to a computer programproduct comprising executable code that when executed, causes executionof functions of the method.

Although the various aspects, embodiments and features of the inventionare recited independently, it should be appreciated that allcombinations of the various aspects, embodiments and features of theinvention are possible and within the scope of the present invention asclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail bymeans of exemplary embodiments with reference to the attached drawings,in which

FIG. 1 shows a simplified block diagram illustrating exemplary systemarchitecture;

FIG. 2 illustrates simulation results for an AAS antenna with 24elements;

FIG. 4 shows a messaging diagram illustrating an exemplary messagingevent according to an embodiment of the invention.

FIG. 5 shows a schematic diagram of a flow chart according to anexemplary embodiment of the invention;

FIG. 6 shows a simplified block diagram illustrating exemplaryapparatuses.

FIG. 7 is a block diagram of an apparatus according to an embodiment ofthe invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

An exemplary embodiment relates to an interface between an RF module(RRH, AAS etc.) and a system module (baseband processing etc.),especially to a case where a receiver has been equipped with a largenumber of antennas.

Currently, the interface between the units is carrying I/Q data which isthen further processed in the system module. The interface may become anissue when the number of Rx antennas becomes large. For example, theantenna may be an AAS antenna with 24 Rx elements.

When designing the interface, it may be desirable to obtain the mostgeneral feature supporting the largest set of features in the future asit is not easy to change the interface. Here three specific constraintsare set: channel estimation, CoMP, and user-specific beamforming.Channel estimation may use both RS slots in a subframe (Rel. 10 andUL-MIMO with OCC). CoMP uses an antenna signal from several RRHs. CoMPhas a possibility for IRC, Turbo Eq. etc. receiver in the system module.User-specific beamforming utilizes per user optimized spatialprocessing, and requires channelling each antenna signal to channelestimation.

Other requirements impacting the interface include data raterequirements and latency requirements. The latency requirementsoriginate from timing required for ack/nack reporting (PHICH). Thus,delaying data transfer leads to a higher data rate requirement due to areduced time for transmission.

Currently, main interface options include CPRI and OBSAI (RP3)interfaces carrying I/Q data from the antennas. Basically, consideringan exemplary AAS antenna with 24 Rx elements, the user specificbeamforming requires 24 antenna streams to be transmitted to the systemmodule, which is clearly too much. Questions related to interfacecompression have been emerging.

A post-DFT interface has been proposed as an alternate solution toreduce a sample rate by a ratio of 1200/2048. Each other aspect issimilar to the above interface.

Another option is to consider antenna combining in RF and transmitmatched filter samples to the system module. The data rates aresignificantly reduced and each constraint in the previous section isstill fulfilled. There is the need to wait for the channel estimatebefore the antenna combining may be carried out and transmission maystart. Thus, the time available for data transfer becomes limited. Thiscreates a connection between DSP processing capabilities (MIPSrequirement) and the data rate requirement in the interface. Thus, apotentially large compression but additional constraints for DSPprocessing requirement may be needed.

An exemplary embodiment relates to a signal subspace based post-FFTinterface. FIG. 1 shows a simplified block diagram illustratingexemplary system architecture, wherein the signal subspace selectionconcept is illustrated.

An exemplary embodiment involves a remote unit (RRH, RF) 203 and acentral unit (system module, central processing unit) 206 and aninterface 205 between them. The remote unit 203 obtains (N) signalstreams 202 from antenna elements/beams 201. (N refers to the number ofstreams obtained from the antenna elements; another option is that Nrefers to the number of fixed beams created before the processing).Within the remote unit 203 there is a subspace selection/calculationunit 204 which selects a signal subspace for each user (or at leastseveral subspaces in a cell) of a dimension M. The interface 205transmits M streams of post-FFT data with a capability to transmit adifferent subspace for different frequency bins (or groups of bins,PRB).

In an exemplary embodiment, a user-specific signal subspace is selectedfor the user. In an exemplary embodiment, a signal subspace is selectedfor one or more users. In an exemplary embodiment, several signalsubspaces are selected in a cell.

Post-FFT samples and frequency selective signal subspaces are detectableissues in the interface. The (N) streams obtained from the antenna maythemselves be the signals from individual antenna elements, groups ofelements or separate antenna beams.

There are several methods to determine the subspace for the interface.As an example, two approaches are listed here which both may beimplemented with the structure described herein. Firstly, in antennaelement/beam selection (MAAS), the best antennas may be selected for theuser depending on the fading conditions (for example, such that thesubspace selection adapts to fast fading, i.e. the subspace selectionadapts to momentary channel state information). Secondly, signalsubspace determination may be based on longer term signal properties(for example, the subspace selection may be based on spatial correlationproperties of the signal, e.g. the expected direction of the arrivedsignal).

An advantage includes significantly reduced data rate for the interface.Also, the number of streams in the interface becomes independent on theantenna design by introducing the subspace selection scheme.

Thus, by using post-DFT (discrete Fourier transform) data it is possibleto introduce the different subspace/beam for the different frequencybins allowing the same data rate towards the system module regardless ofwhether the user-specific subspaces have been used or not.

In FIG. 2, exemplary simulation results are shown. The antenna used inthe simulation is an AAS with 24 elements. A long term subspaceselection scheme was used to select either 4 or 8 streams for each UEseparately, 24/4 UES and 24/8 UES. It was found that the simulatedscheme has the best performance over the fixed antenna beam solutionswith the same number of streams.

Additionally, in some simulation cases it has been found that the userspecific subspace selection may reduce the number of streams requiredfor the system module for the same reference performance.

There may be several possibilities for the subspace selection and theabove result is only an example of one scheme in one scenario.

In the following, different embodiments will be described using, as anexample of a system architecture whereto the embodiments may be applied,an architecture based on LTE/LTE-A network elements, without restrictingthe embodiment to such an architecture, however. The embodimentsdescribed in these examples are not limited to the LTE/LTE-A radiosystems but can also be implemented in other radio systems, such as UMTS(universal mobile telecommunications system), GSM, EDGE, WCDMA,Bluetooth network, WLAN or other fixed, mobile or wireless network. Inan embodiment, the presented solution may be applied between elementsbelonging to different but compatible systems such as LTE and UMTS.

A general architecture of a communication system is illustrated in FIG.6. FIG. 6 is a simplified system architecture only showing some elementsand functional entities, all being logical units whose implementationmay differ from what is shown. The connections shown in FIG. 6 arelogical connections; the actual physical connections may be different.It is apparent to a person skilled in the art that the systems alsocomprise other functions and structures. It should be appreciated thatthe functions, structures, elements and the protocols used in or fordetermining a signal subspace, are irrelevant to the actual invention.Therefore, they need not to be discussed in more detail here.

The exemplary radio system of FIG. 6 comprises a network node 601 of anetwork operator. The network node 601 may include e.g. an LTE basestation of a macro cell (eNB), radio network controller (RNC), or anyother network element, or a combination of network elements. The networknode 601 may be connected to one or more core network (CN) elements (notshown in FIG. 6) such as a mobile switching centre (MSC), MSC server(MSS), mobility management entity (MME), gateway GPRS support node(GGSN), serving GPRS support node (SGSN), home location register (HLR),home subscriber server (HSS), visitor location register (VLR). In FIG.6, the radio network node 601 that may also be called eNB (enhancednode-B, evolved node-B) or network apparatus of the radio system, hoststhe functions for radio resource management in a public land mobilenetwork.

FIG. 6 shows one or more user equipment 602 located in the service areaof the radio network node 601. The user equipment refers to a portablecomputing device, and it may also be referred to as a user terminal.Such computing devices include wireless mobile communication devicesoperating with or without a subscriber identification module (SIM) inhardware or in software, including, but not limited to, the followingtypes of devices: mobile phone, smart-phone, personal digital assistant(PDA), handset, laptop computer. In the example situation of FIG. 6, theuser equipment 602 is capable of connecting to the radio network node601 via a (cellular radio) connection 603.

FIG. 7 is a block diagram of an apparatus according to an embodiment ofthe invention. FIG. 7 shows a user equipment 602 located in the area ofa radio network node 601. The user equipment 602 is configured to be inconnection 603 with the radio network node 601. The user equipment or UE602 comprises a controller 701 operationally connected to a memory 702and a transceiver 703. The controller 701 controls the operation of theuser equipment 602. The memory 702 is configured to store software anddata. The transceiver 703 is configured to set up and maintain awireless connection 603 to the radio network node 601, respectively. Thetransceiver 703 is operationally connected to a set of antenna ports 704connected to an antenna arrangement 705. The antenna arrangement 705 maycomprise a set of antennas. The number of antennas may be one to four,for example. The number of antennas is not limited to any particularnumber. The user equipment 602 may also comprise various othercomponents, such as a user interface, camera, and media player. They arenot displayed in the figure due to simplicity.

The radio network node 601, such as an LTE (or LTE-A) base station(eNode-B, eNB) comprises a controller 706 operationally connected to amemory 707, and a transceiver 708. The controller 706 controls theoperation of the radio network node 601. The memory 707 is configured tostore software and data. The transceiver 708 is configured to set up andmaintain a wireless connection to the user equipment 602 within theservice area of the radio network node 601. The transceiver 708 isoperationally connected to an antenna arrangement 709. The antennaarrangement 709 may comprise a set of antennas. The number of antennasmay be two to four, for example. The number of antennas is not limitedto any particular number. The radio network node 601 may beoperationally connected (directly or indirectly) to another networkelement of the communication system, such as a further radio networknode, radio network controller (RNC), a mobility management entity(MME), an MSC server (MSS), a mobile switching centre (MSC), a radioresource management (RRM) node, a gateway GPRS support node, anoperations, administrations and maintenance (OAM) node, a home locationregister (HLR), a visitor location register (VLR), a serving GPRSsupport node, a gateway, and/or a server, via an interface (not shown inFIG. 7). The embodiments are not, however, restricted to the networkgiven above as an example, but a person skilled in the art may apply thesolution to other communication networks provided with the necessaryproperties. For example, the connections between different networkelements may be realized with internet protocol (IP) connections.

Although the apparatus 601, 602 has been depicted as one entity,different modules and memory may be implemented in one or more physicalor logical entities. The apparatus may also be a user terminal which isa piece of equipment or a device that associates, or is arranged toassociate, the user terminal and its user with a subscription and allowsa user to interact with a communications system. The user terminalpresents information to the user and allows the user to inputinformation. In other words, the user terminal may be any terminalcapable of receiving information from and/or transmitting information tothe network, connectable to the network wirelessly or via a fixedconnection. Examples of the user terminals include a personal computer,a game console, a laptop (a notebook), a personal digital assistant, amobile station (mobile phone), a smart phone, and a line telephone.

The apparatus 601, 602 may generally include a processor, controller,control unit or the like connected to a memory and to variousinter-faces of the appartus. Generally the processor is a centralprocessing unit, but the processor may be an additional operationprocessor. The processor may comprise a computer processor,application-specific integrated circuit (ASIC), field-programmable gatearray (FPGA), and/or other hardware components that have been programmedin such a way to carry out one or more functions of an embodiment.

The memory 702, 707 may include volatile and/or non-volatile memory andtypically stores content, data, or the like. For example, the memory702, 707 may store computer program code such as software applications(for example, for the detector unit and/or for the adjuster unit) oroperating systems, information, data, content, or the like for aprocessor to perform steps associated with operation of the apparatus inaccordance with embodiments. The memory may be, for example, randomaccess memory (RAM), a hard drive, or other fixed data memory or storagedevice. Further, the memory, or part of it, may be removable memorydetachably connected to the apparatus.

The techniques described herein may be implemented by various means sothat an apparatus implementing one or more functions of a correspondingentity described with an embodiment comprises not only prior art means,but also means for implementing the one or more functions of acorresponding apparatus described with an embodiment and it may compriseseparate means for each separate function, or means may be configured toperform two or more functions. For example, these techniques may beimplemented in hardware (one or more apparatuses), firmware (one or moreapparatuses), software (one or more modules), or combinations thereof.For a firmware or software, implementation can be through modules (e.g.procedures, functions, and so on) that perform the functions describedherein. The software codes may be stored in any suitable,processor/computer-readable data storage medium(s) or memory unit(s) orarticle(s) of manufacture and executed by one or moreprocessors/computers. The data storage medium or the memory unit may beimplemented within the processor/computer or external to theprocessor/computer, in which case it can be communicatively coupled tothe processor/computer via various means as is known in the art.

The signalling chart of FIG. 3 illustrates the required signalling. Inthe example of FIG. 3, in item 301, an apparatus 203 which may comprisee.g. a remote unit (a remote apparatus e.g. a radio frequency unitand/or a remote radio head unit) implemented in a base station, may, initem 301, obtain signal streams from antenna elements or antenna beams.In item 302, the apparatus may, based on the obtained signal streams,select a signal subspace for a user, the signal subspace having adimension M. Specifically, the number of the signal streams obtained initem 301 may be different from the (selected) dimension M of theselected signal subspace. In item 303, the apparatus may, based on theselected signal subspace, transmit, via an interface 205 to a centralapparatus 206, M streams of post-fast-Fourier-transform data. Thus theinterface 205 between the remote unit 203 and the central apparatus 206is capable of transmitting a different subspace for different frequencybins. The central apparatus 206 may comprise e.g. a central unit (e.g. acentral processing unit and/or a system module) implemented in the basestation. In item 304, the central unit 206 may receive the M streams ofpost-fast-Fourier-transform data.

FIG. 4 is a flow chart illustrating an exemplary embodiment. Anapparatus 203 which may comprise e.g. a remote unit (a remote apparatuse.g. a radio frequency unit and/or a remote radio head unit) implementedin a base station, may, in item 401, obtain signal streams from antennaelements or antenna beams. In item 402, the apparatus may, based on theobtained signal streams, select a signal subspace for a user, the signalsubspace having a dimension M. Specifically, the number of the signalstreams obtained in item 401 may be different from the (selected)dimension M of the selected signal subspace. In item 403, the apparatusmay, based on the selected signal subspace, transmit, via an interface205 to a central apparatus 206, M streams of post-fast-Fourier-transformdata. Thus the interface 205 between the remote unit 203 and the centralapparatus 206 is capable of transmitting a different subspace fordifferent frequency bins. The central apparatus 206 may comprise e.g. acentral unit (e.g. a central processing unit and/or a system module)implemented in the base station.

FIG. 5 is a flow chart illustrating an exemplary embodiment. Anapparatus 206 which may comprise e.g. a central unit (a centralapparatus e.g. a central processing unit and/or a system module)implemented in a base station, may, in item 501, receive M streams ofpost-fast-Fourier-transform data transmitted, via an interface 205 froman apparatus 203 which may comprise e.g. a remote unit (a remoteapparatus e.g. a radio frequency unit and/or a remote radio head unit)implemented in a base station. M represents a dimension of a signalsubspace selected in the remote unit 203 for a user.

In an exemplary embodiment, there is a group of frequency bins, andthere may be several groups of frequency bins within FFT. Thus, a signalsubspace may be frequency bin specific or frequency bin group specific,or the signal subspace may be specific to several groups of frequencybins. Therefore, the interface 205 may be capable of transmitting afrequency bin specific signal subspace to the frequency bin, capable oftransmitting a frequency bin group specific signal subspace to thefrequency bin group, and/or capable of transmitting a multiple frequencybin group specific signal subspace to the multiple frequency bin groups.

In an exemplary embodiment, post-FFT data refers to data that is dividedto individual samples comprising information on each user for whichindividual subspace definitions may be applied. The users are in afrequency domain, and FFT (or DFT) performs the division.

In an exemplary embodiment, the dimension M is selected according toequipment capabilities and/or interface capabilities.

An exemplary embodiment may be implemented as a computer programcomprising instructions for executing a computer process for activeantenna array beam calibration. The computer program may be stored on acomputer program distribution medium readable by a computer or aprocessor. The computer program medium may be, for example but notlimited to, an electric, magnetic, optical, infrared or semiconductorsystem, device or transmission medium. The computer program medium mayinclude at least one of the following media: a computer readable medium,a program storage medium, a record medium, a computer readable memory, arandom access memory, an erasable programmable read-only memory, acomputer readable software distribution package, a computer readablesignal, a computer readable telecommunications signal, computer readableprinted matter, and a computer readable compressed software package.

The steps/points, signalling messages and related functions de-scribedabove in FIGS. 1 to 7 are in no absolute chronological order, and someof the steps/points may be performed simultaneously or in an orderdiffering from the given one. Other functions can also be executedbetween the steps/points or within the steps/points and other signallingmessages sent be-tween the illustrated messages. Some of thesteps/points or part of the steps/points can also be left out orreplaced by a corresponding step/point or part of the step/point. Theapparatus operations illustrate a procedure that may be implemented inone or more physical or logical entities. The signalling messages areonly exemplary and may even comprise several separate messages fortransmitting the same information. In addition, the messages may alsocontain other information.

It will be obvious to a person skilled in the art that, as thetechnology advances, the inventive concept can be implemented in variousways. The invention and its embodiments are not limited to the examplesdescribed above but may vary within the scope of the claims.

LIST OF ABBREVIATIONS

PRB physical resource block

RF radio frequency

CPU central processing unit

RRH remote radio head

AAS active antenna system

MAAS multiband active antenna system

I/Q in-phase/quadrature

Rx receiver

DSP digital signal processing

MIPS million of instructions per second

CPRI common public radio interface

OBSAI open base station architecture initiative

RP3 reference point 3

MIMO multiple input multiple output

OCC outgoing call control

CoMP coordinated multipoint transmission

RS reference signal

UL uplink

IRC interference rejection combining

PHICH physical hybrid automatic repeat request indicator channel

ack acknowledgement

nack negative acknowledgement

1. A method for determining a signal subspace in a communicationssystem, the method comprising obtaining, in a remote apparatus, signalstreams from antenna elements or signal streams from antenna beams;based on the obtained signal streams, selecting a signal sub-space for auser, the signal subspace having a dimension M; and based on theselected signal subspace, transmitting, via an interface to a centralapparatus, M streams of post-fast-Fourier-transform data, the interfacebeing capable of transmitting a different signal subspace for differentfrequency bins.
 2. A method according to claim 1, characterized in thatsaid selecting comprises selecting a signal subspace for one or moreusers.
 3. A method according to claim 1, characterized in that saidselecting comprises selecting several signal subspaces in a cell.
 4. Amethod as claimed in claim 1, characterized in that the interface iscapable of transmitting different signal subspaces for different groupsof frequency bins.
 5. A method as claimed in claim 1, characterized inthat the interface is capable of detecting post-fast-Fourier-transformsamples and frequency selective signal subspaces.
 6. A method as claimedin claim 1, characterized in that the obtained signal streams comprisesignals from individual antenna elements, signals from groups of antennaelements, or signals from separate antenna beams.
 7. A method as claimedin claim 1, characterized by selecting the antenna element or antennabeam for the user based on fading conditions.
 8. A method as claimed inclaim 1, characterized by performing signal subspace determination basedon longer term signal properties.
 9. An apparatus comprising at leastone processor; and at least one memory including a computer programcode, wherein the at least one memory and the computer program code areconfigured to, with the at least one processor, cause the apparatus toobtain signal streams from antenna elements or signal streams fromantenna beams; based on the obtained signal streams, select a signalsubspace for a user, the signal subspace having a dimension M; and basedon the selected signal subspace, transmit, via an interface to a centralapparatus, M streams of post-fast-Fourier-transform data, the interfacebeing capable of transmitting a different signal subspace for differentfrequency bins.
 10. An apparatus according to claim 9, characterized inthat said selecting comprises selecting a signal subspace for one ormore users.
 11. An apparatus according to claim 9, characterized in thatsaid selecting comprises selecting several signal subspaces in a cell.12. An apparatus as claimed in claim 9, characterized in that theinterface is capable of transmitting different signal subspaces fordifferent groups of frequency bins.
 13. An apparatus as claimed in claim9, characterized in that the interface is capable of detectingpost-fast-Fourier-transform samples and frequency selective signalsubspaces.
 14. An apparatus as claimed in claim 9, characterized in thatthe obtained signal streams comprise signals from individual antennaelements, signals from groups of antenna elements, or signals fromseparate antenna beams.
 15. An apparatus as claimed in claim 9,characterized in that the at least one memory and the computer programcode are configured to, with the at least one processor, cause theapparatus to select the antenna element or antenna beam for the userbased on fading conditions.
 16. An apparatus as claimed in claim 9,characterized in that the at least one memory and the computer programcode are configured to, with the at least one processor, cause theapparatus to perform signal sub-space determination based on longer termsignal properties.
 17. An apparatus as claimed in claim 9, characterizedin that it comprises a remote radio head unit or a radio frequency unit.18. A base station comprising an apparatus as claimed in claim
 9. 19. Acomputer program product comprising executable code that when executed,causes execution of functions of a method according to claim 1.